1. Technical Field
The present invention relates to electric double layer capacitors having large capacity and high power, and in particular, relates to a technique to prevent decomposition of the electrolyte solution used in the capacitor.
2. Background Art
An electric double layer capacitor has characteristics such as long service life, high cycling characteristics, and characteristics of charge and discharge with large current since there are no chemical reactions during charge and discharge of the capacitor as there are in a conventional secondary battery. Therefore, this type of capacitor is attracting much attention as a new type of storage battery or as a driving power supply for automobiles and devices. In particular, electric double layer capacitors having large capacity and high power are being developed.
As an example of such an electric double layer capacitor, a button-type electric double layer capacitor is shown in FIG. 1. As shown in FIG. 1, the capacitor 1 includes a case 2, a pair of polarizing electrodes 3 and 4 contained in the case 2, a spacer 5 disposed between the electrodes, and electrolyte solution filled in the case 2. The case 2 comprises an aluminum body 7 having an opening part 6 and an aluminum cover plate 8 which closes the opening part 6. A part between the outer circumference of the cover plate 8 and inner circumference of the body 7 is sealed with a sealing material 9. The polarizing electrodes 3 and 4 comprise a mixture of an activated carbon for an electrode, conductive filler, and binder.
Conventionally, as the electrolyte solution used as a material of the electric double layer capacitor, a water-based electrolyte solution and a non-water-based electrolyte solution can be mentioned. In particular, since high voltage use is required to improve energy density of the electric double layer capacitor, the non-water-based electrolyte solution which can be charged and discharged at relatively high voltage has been widely used. As the non-water-based electrolyte solution, the solution in which various conditions such as low-temperature property, solubility of salt, dielectric constant, safety, electrolyte solution decomposition property, boiling point, cost and the like is improved is required. As a material which meets these requirements, an electrolyte solution in which propylene carbonate is mainly used as a solvent and a quaternary ammonium salt is added as a supporting salt, may be mentioned.
However, in the electric double layer capacitor having an electrolyte solution containing propylene carbonate and an alkali activated carbon, in the case in which it is charged and discharged repeatedly under high voltage, the electrolyte solution is gradually decomposed by an electric current and gas is generated. The inner pressure of the capacitor is increased by the gas generation, and the capacitor case may be deformed or broken. Furthermore, as a result of consumption of the electrolyte solution by the electrolysis, capacitance may be deteriorated and internal resistance may be increased. Furthermore, the viscosity of the electrolyte solution in the conventional capacitor is increased at low temperatures, and the property of charging and discharging at low temperatures may be deteriorated.
To solve such problems, a method in which various types of materials are added to reduce decomposition of the electrolyte solution, a method to reduce internal resistance, and the like have been suggested. Practically, a method in which a fluorine substituted type carbonic acid ester is added to a electrolyte solution having an organic carbonate as a solvent, and support salt (see Japanese Unexamined Patent Application Publication No. 2000-150317) and a method in which γ-butyrolactone or γ-valerolactone is added (see Japanese Unexamined Patent Application Publication No. 2001-217150) have been disclosed. Furthermore, a low viscosity solvent, for example, a chain carbonate such as dimethylcarbonate, ethylmethylcarbonate or the like, an aliphatic monocarboxylic acid ester such as γ-butyrolactone, γ-valerolactone or the like (see Japanese Unexamined Patent Application Publication No. Hei 8-236404, No. Hei 8-273936), a solvent in which ethylenecarbonate is mixed with propylenecarbonate (see Japanese Unexamined Patent Application Publication No. 2000-208372) have been suggested.
However, in the technique disclosed in No. 2000-150317, the fluorine substituted type carbonic acid ester is expensive. In the technique disclosed in No. 2001-217150, the effect of restraining decomposition of the electrolyte solution by γ-butyrolactone or γ-valerolactone is insufficient. In addition, the two techniques had been completed depending on assumption of efficiency improvement at high temperatures; therefore, improvement of charging and discharging property at low temperatures is not sufficient. Furthermore, in the solvents disclosed in No. Hei 8-236404, No. Hei 8-273936, and No. 2000-208372, internal resistance of the capacitor is improved; however, reduction of gas generation is not sufficient, the same amount or a greater amount of gas is generated compared to the case of a conventional propylene carbonated solvent.